1) Field of the Invention
The present invention relates to an optical scanner used for laser-beam printers (LBP), regular paper facsimile systems (PPF), digital copying machines and the like, and an image formation apparatus that employs such an optical scanner.
2) Description of the Related Art
High density, such as 1200 to 2400 dpi (dots per inch) has recently been required as the scanning density in optical scanners. In order to achieve high density in optical scanning, it is necessary to decrease the beam diameter of optical beams condensed on a surface to be scanned. While a requirement for decreasing the beam diameter increases as the optical scanner has a higher density, it is also required to manufacture the optical scanner at a low cost. In order to deal with this requirement for low cost, a lens made of resin is often used for the scanning lenses. However, in the resin lens, image-forming misregistration is large due to a temperature change, and it is difficult to decrease the beam diameter.
As an apparatus in which such image formation misregistration is suppressed, there are known apparatus disclosed in U.S. Pat. No. 2,736,984 (Publication 1), and U.S. Pat. No. 2,804,647 (Publication 2).
The apparatus in the Publication 1 has following components. That is, a collimator comprising a semiconductor laser, a collimator lens and a retaining member which fixes and retains these components, and a scanning image formation optical system which forms an image of a light flux from the collimator deflected by a deflection unit on a photosensitive material. By optimizing a coefficient of linear expansion, a refractive index or the like of the collimator lens, the scanning image formation optical system and the retaining member, the image formation misregistration of the whole optical system is reduced.
The apparatus in the Publication 2 comprises following components. That is, a first image formation optical system which forms an image of light from a light source linearly, and a second image formation optical system which allows optical beams deflected by a deflection apparatus having a deflecting reflective surface to condense on a surface to be scanned, at an image formation position of the first image formation optical system. By using a resin lens (plastic lens) having a negative power for the first image formation optical system, an image formation misregistration which occurs in the second image formation optical system is cancelled, to thereby reduce the image formation misregistration of the whole optical system.
As the optical scanning speed of the optical scanner increases, it becomes necessary to rotate a deflector such as a polygon scanner at a high speed. Therefore, the temperature in the vicinity of the deflector differs from the temperature in the vicinity of a position away from the deflector, due to heat generated by the deflector which rotates at a high speed, and hence the temperature distribution in the optical scanner, that is, in an optical housing, becomes non-uniform. As a result, it is necessary to take measures for suppressing image-forming misregistration, on the assumption that the temperature distribution in the optical scanner is non-uniform.
However, in any of the Publications 1 and 2, it is not described nor suggested that image formation misregistration is suppressed on the assumption that the temperature distribution in the optical scanner (that is, in the optical housing) is non-uniform. In other words, the optical scanners described in the respective Publications 1 and 2 do not suppress image formation misregistration, on the assumption that the temperature distribution in the optical housing is non-uniform, due to the heat generated by the deflector which rotates at a high speed. Thus, the optical scanners described in the respective Publications 1 and 2 have a problem in that even if measures as described in each publication are taken, the beam diameter of the optical beam and a pitch deflection of a vertical scanning beam pitch of multi-beam are deteriorated.
In the optical scanner in the Publication 2, as described above, a resin lens having a negative power is used for the first image formation optical system, so that an image formation misregistration which occurs in the second image formation optical system is cancelled. However, in order to improve the correction result of the image formation misregistration, it is necessary to set the negative power of the resin lens large. Thereby, machining of the resin lens becomes difficult, and there is another problem in that the wave front aberration is deteriorated.